This invention relates to a valve and, more particularly, to a pressure relief valve.
Pressurized lines are commonly fitted with either rupture discs or pressure relief valves to prevent the pressure of fluid inside the lines from exceeding a prescribed limit beyond which the line or equipment connected thereto could rupture or be damaged. Rupture discs are relatively simple, comprising a simple disc which interfaces with fluid in a line. The disc comprises a skin, the thickness of which is such that when fluid pressure in a line exceeds a predetermined pressure, the disc ruptures, thereby relieving the pressure. An obvious drawback to such a rupture disc is that once pressure is relieved, there is no provision for the area of the rupture to be closed off so that normal fluid pressure may be maintained. Consequently, an entire line and often an entire system must be shut down to replace the ruptured disc.
To cure this drawback with rupture discs, particularly where it may be anticipated that the line pressure may frequently exceed the design pressure or where it is undesirable to shut down an entire system to replace such a disc, pressure relief valves (PRVs) were developed. Rather than using a disc that ruptures when pressure exceeds a predetermined pressure, PRVs utilize a valve mechanism that is maintained in a closed position by a compressed spring. The PRV includes an inlet and an outlet for fluid to flow into and out of. The inlet is connected in fluid communication with a tee in the line and the outlet is connected to an exhaust or overflow line. The compression of the spring is adjusted to keep the valve in its closed position until a pressure exceeding a predetermined pressure is impressed upon it. Once fluid on the inlet side attains a pressure which exceeds the predetermined pressure, the valve mechanism moves to an open position, fluid flows into the inlet through the PRV and out through the outlet, thereby relieving the pressure. Once the pressure is reduced to a point below the predetermined pressure, the spring moves the valve mechanism back to its closed position, and normal fluid flow through the line resumes.
There are, however, several drawbacks associated with such PRVs. For example, because there is only a single inlet, fluid does not flow through the inlet unless pressure is being relieved by the PRV. Consequently, product deposits may accumulate in the inlet and, as a consequence, plug the PRV, thereby requiring periodic maintenance of the PRV and the potential shut down of associated systems. This drawback is particularly acute in lines carrying viscous fluids that solidify at low temperatures, though the severity of this drawback may be abated somewhat by applying heat, e.g., steam tracing, the inlet to prevent the temperature of the fluid therein from falling too low. In some industries, such as the pharmaceutical, food, and beverage industries, a PRV may require maintenance on a daily basis or even more often depending on when a product used in a line is changed. Such maintenance typically requires that the PRV be removed from the line and, upon re-assembly of the PRV, that the compression of the PRV spring be re-adjusted to meet close tolerances and certified accordingly.